1. Field of the Invention
The present invention generally relates to electric lamps, and more particularly, to electric lamps having a light source with a transparent heat conserving coating.
2. Description of the Related Art
Metal halide arc discharge lamps are frequently employed because of their high luminous efficacy and long life. A typical metal halide arc discharge lamp includes a quartz lamp capsule or arc tube having a bulbous portion containing fill material. The fill material is typically mercury, a metal halide to improve efficacy and color, and a rare gas to facilitate starting. Tungsten electrodes are sealed in opposite ends of the arc tube to enable energization of a discharge arc within the arc tube.
The initial application of an excitation source across the electrodes causes the rare gas to ionize and produce light. Continued application of the excitation source causes the vaporization and the ionization of the mercury and metal halide to produce light. Due to internal convection a nonuniform temperature distribution occurs in the arc tube bulbous portion. When the arc tube is horizontally disposed the top of the bulbous portion or top wall becomes overheated causing a hot spot or hot region having a temperature hotter than the average temperature of the bulbous portion. Overheating of the arc tube can cause both devitrification of the quartz and reaction of the quartz with the metal halide. In either case the quartz arc tube splits causing premature failure of the lamp.
When the arc tube is horizontally disposed, the bottom of the bulbous portion or bottom wall also becomes relatively cool causing a cold spot or cold region having a temperature colder than an average temperature of the bulbous portion. It is known that metal halides migrate to the coolest spot of the arc tube. Therefore, the metal halides typically migrate to the cold region at the bottom of the bulbous portion and condense on the bottom wall of the arc tube. The metal halides can also migrate to and condense at cold regions at ends of the bulbous portion, a location furthest from the arc discharge. Condensation of the metal halide results in a lower luminous efficacy and color.
It is known to use an infrared reflecting film covering the entire outer surface of the bulbous portion to improve the efficacy of the lamp. For example, see U.S. Pat. Nos. 5,221,876, 5,017,839, and 4,987,343, the disclosures of which are expressly incorporated by reference herein in their entirety. The efficacy of the lamp is improved by reflecting infrared energy emitted by the lamp back substantially toward the arc so that the arc temperature may be increased and maintained without any increase in input power from the excitation source. The infrared reflecting film, however, operates to increase the temperature of the entire bulbous portion. The temperatures of the hot regions as well as the cold regions are increased.
It is also known to use an opaque and diffuse end coat on the bulbous portion. For example, see U.S. Pat. No. 3,325,662 disclosing a calcium pyrophosphate coating and U.S. Pat. No. 3,374,377 disclosing a zirconium oxide coating, the disclosures of which are expressly incorporated by reference herein in their entirety. The end coat absorbs infrared radiation to act as a heat trap. If the lamp is intended to be operated vertically only the bottom end is coated. If the lamp is intended to be operated vertically or horizontally both ends are coated. Because the end coat is opaque most of the visible radiation is reflected back, however, part is absorbed and illuminous output of the lamp is reduced by about 5 to 10%. Additionally, the visible radiation that is reflected by the end coat is reflected in a uncontrolled manner causing stray light. The stray light results in glare that is undesirable in optically controlled applications. Furthermore, the end coat is applied by dipping or spraying which results in low repeatability. The height of the end coat varies about 1 millimeter. This is undesirable because the performance of the metal halides, and thus the efficacy of the lamp, is closely connected to the height of the end coat.
U.S. Pat. No. 4,307,315, the disclosure of which is expressly incorporated by reference herein in its entirety, discloses a high pressure discharge lamp having a quartz vessel. A portion of the vessel is ultraviolet radiation absorbing quartz glass and the remainder is ultraviolet radiation transmitting quartz. Fabrication of the vessel from doped and undoped quartz pieces is laborious and impractical for arc tubes of a small size.
Accordingly, a need exists for a high intensity light source having a transparent coating that imposes an additional thermal load at cold regions so that high metal halide vapor pressure can be maintained. It would be desirable to have a coating that does not raise the temperature of the hot region and can be applied in a tightly controlled and economical manner.